Could we contact an alien SETI?

[nzric]

Could someone tell me what is the link between distance in space, strength of a radio signal, and size of any listening device? Considering the size/sensitivity of SETI, how close would any alien civilisation need to be for us to detect unusual radio signals if they are not transmitting to us, i.e. if the planet just emitted a similar kind & strength of radio chatter to our own planet?

Likewise, if an alien civilisation somehow knew we had a SETI program, the size/sensitivity of the system, and when we would be pointing the dish directly at their planet, how strong would their signal need to be vs their distance away from our planet, in order for us to receive it? Nothing fancy, just a string of prime numbers.

Has anyone looked at this in detail? i.e. the number of stars within range where we could detect a signal with the current SETI setup if they transmitted a similar kind of signal that we would use, vs the amount of remaining stars that are at a distance where any extraterrestrials would need to use some extraordinary transmission strength (or we would need a huge and extremely sensitive receptor) in order to make contact?

 

[EHocking]

Could someone tell me what is the link between distance in space, strength of a radio signal, and size of any listening device? Considering the size/sensitivity of SETI, how close would any alien civilisation need to be for us to detect unusual radio signals if they are not transmitting to us, i.e. if the planet just emitted a similar kind & strength of radio chatter to our own planet?

Likewise, if an alien civilisation somehow knew we had a SETI program, the size/sensitivity of the system, and when we would be pointing the dish directly at their planet, how strong would their signal need to be vs their distance away from our planet, in order for us to receive it? Nothing fancy, just a string of prime numbers.

Has anyone looked at this in detail? i.e. the number of stars within range where we could detect a signal with the current SETI setup if they transmitted a similar kind of signal that we would use, vs the amount of remaining stars that are at a distance where any extraterrestrials would need to use some extraordinary transmission strength (or we would need a huge and extremely sensitive receptor) in order to make contact?

This introduction to Targeted Star Search at the SETI website, probably goes some way to begin to explain your questions.

In short, they target 100 to 200 sun-like stars that are 25-200 light years from earth, i.e., stars of a similar age to our sun.
The searched radio bandwidth was narrowed, and the search is for patterns and certainly initially for repetitions of prime numbers (Project Ozma)

 

[Tim Thompson]

Seti

Could someone tell me what is the link between distance in space, strength of a radio signal, and size of any listening device?

The strength of a broadcast signal will fall off as the square of the distance. So pick some arbitrary standard distance, say 1 light year. The signal strength at 2 light years will be 1/4 of what it was at 1 light year (1/2² = 1/4). At 10 light years the signal will be 1% of what it was at 1 light year (1/10² = 1/100) and so forth. The way to get around this is to start with a really big signal and the easiest way to do that is to transmit a very narrow angle very powerful beam. In similar fashion, the effectiveness of a receiver antenna depends on its area, as well as the sensitivity of the signal detector attached to the antenna. So you want the biggest receiver antenna you can get with the most sensitive signal detector attached to it.

Considering the size/sensitivity of SETI, how close would any alien civilisation need to be for us to detect unusual radio signals if they are not transmitting to us, i.e. if the planet just emitted a similar kind & strength of radio chatter to our own planet?

I don't remember the numbers (I once worked on the NASA SETI project at JPL), but it's not far. Say 20 light years or so, after that the signal strength is just too weak for normal efforts. But I guess if someone wanted to bust the bank and look really hard, you could stretch that out.

Likewise, if an alien civilisation somehow knew we had a SETI program, the size/sensitivity of the system, and when we would be pointing the dish directly at their planet, how strong would their signal need to be vs their distance away from our planet, in order for us to receive it? Nothing fancy, just a string of prime numbers.

The system that was in place at Arecibo 30 years ago could have detected "itself" (i.e., another similar system in transmission mode) from anywhere in our Milky Way galaxy, at least 100,000 light years distant. I presume the system has not deteriorated since then, so it is likely still true (maybe even more true) today than it was then. Their main antenna is 1000 feet (305 meters) in diameter, but with limited pointing capability due to its large size and being fixed to the ground. It is "just" an engineering problem to make an arbitrarily larger, and hence more sensitive antenna. So with some engineering work, some system could be made to communicate across galactic distances (i.e., million of light years), at least in principle.

Has anyone looked at this in detail? i.e. the number of stars within range where we could detect a signal with the current SETI setup if they transmitted a similar kind of signal that we would use, vs the amount of remaining stars that are at a distance where any extraterrestrials would need to use some extraordinary transmission strength (or we would need a huge and extremely sensitive receptor) in order to make contact?

Oh yes, this was all hashed out in gory detail decades ago, when there was apparently a lot more interest in SETI than there is now. There was always a lot of opposition to spending any money on SETI projects, even when there arguably was money available to spend, but in today's more depressed economy, it's even worse. I don't think we will ever see another tax payer supported SETI project, at least for the next several decades.